A. Field of the Invention
The present invention relates to methods and apparatus for making dental models. More particularly, the invention relates to an improved method and apparatus for producing crowns, bridges, tooth prostheses and other dental restorations.
B. Background of the Invention
Dental models are widely utilized in the fabrication of replacements, or restorations, for missing or damaged teeth. Such models are usually produced in a sequence of well-known steps. First, a negative impression is made by a dentist of the upper or lower jaw of a patient, using a flexible, quick-setting resilient molding material, such as liquid latex rubber poured into a holding device. After the rubber has set, the negative impression and holding device are removed from the patient's mouth and positioned so that the openings of the negative impression face upwards. Thus, an upper jaw impression is removed from the mouth retained in the same orientation, while a lower jaw impression is inverted after removing it from the patient's mouth. A jaw impression in a holder is then filled with a liquid plaster material, which is frequently referred to as die stone. After the die stone has hardened, the casting is removed from the flexible dental impression mold.
The casting, or model as it is usually referred to, is a positive replication of that portion of the jaw initially used to make the negative impression mold. Thus, the model is an accurate replica of the teeth and soft tissue of the upper or lower jaw, or a portion thereof, from which a negative impression has been made.
Dental models made as described above usually have an arched shape and are therefore, frequently referred to as dental arches. Such dental arches are usually fastened to some sort of base plate for convenience in performing further steps described below.
The existing art utilizes several techniques of providing a base plate for dental arches. The techniques fall generally into either of two broad method classifications. One method utilizes a base of the gypsum-like die stone that is used to make the arch, and is formed from liquid die stone, after the dental arch is formed, by pouring the liquid die stone into a box form positioned around the outer perimeter of the already hardened dental arch.
The other method of providing a base for a dental arch consists of attaching the arch to a pre-manufactured plate made of dimensionally stable material and having a generally semi-circular shape, the convex surface of which conforms generally to the convex surface of the dental arch.
Bases are provided for dental arches so that the arch may be segmented into individual sections or dies, by means of saw cuts, and removably returned to the original positions of the sections in the arch. Individual sections, or dies, are utilized in the fabrication of dental restorations including dentures, partial dentures, crowns, jackets, bridges, and implant supported restorations. The dies are used as models for making individual restorations of various materials such as gold and other metals, plastics and various ceramic materials. After a restoration has been fabricated, it must be remounted on the arch support base at the precise position that it was removed from, to verify that the restoration is of the precise shape and size to fit properly in the mouth of the patient.
The method of removably attaching individual dies to the arch support base usually consists of molding two dowel pins into each die position of the dental arch, the dowel pins protruding downwards into corresponding holes in the base. Two dowel pins are used to preclude rotation of a die with respect to the base.
Several methods exist for locating dowel pins in desired positions, typically two per tooth, in each die. The accuracy of placement of pins in each die is critical, since even a small placement error could result in the wall of a die breaking through if a saw cut is too close to a pin. Therefore, the preferred method of molding pins into a dental arch and fitting the pins into a base employs a pre-manufactured, pre-fabricated base made of plastic material having greater dimensional stability than the die stone from which the arch is fabricated.
Several systems exist for pouring a dental model utilizing a pre-fabricated, as opposed to a poured, integral base.
One such system, disclosed in Zeiser, U.S. Pat. No. 4,371,339, Denture Mold And Method Of And Arrangement For Its Manufacture, requires the use of a relatively complicated, heavy and expensive orienting apparatus, manufactured to precise tolerances, for holding a dental impression while determining the locations on a pre-fabricated base plate where holes are subsequently to be made for securing dowel pins which will be molded into a dental arch.
Another method and apparatus for making dental models using pre-fabricated base plates is disclosed in Kiefer, U.S. Pat. No. 4,708,835, Nov. 24, 1987, Method And Apparatus For Making A Dental Model Mounted On A Base Plate. In Kiefer, a pre-fabricated base plate containing a plurality of pre-formed holes is fitted with dowel pins in each location where it is desired to make a die removable from the cast dental arch. Two methods of determining which of the pre-formed holes in a base plate are to have dowel pins inserted in them are disclosed. Both methods require that a dental impression be attached to a carrier plate having guide means for removably receiving a base plate in a precisely repeatable orientation with respect to the dental impression. The carrier plate includes a flat base and a vertical wall or upright which protrudes upwards from the rear edge of the base.
One version of the Kiefer method requires the use of a transparent datum plate which is positioned over the base plate and fitted with marker pins at desired locations. The datum plate is then removed from the carrier plate, flipped over and re-mounted on the opposite side of the upright of the carrier plate. A base plate is then mounted to the upright of the carrier plate, over the datum plate, and dowel pins inserted into pre-formed holes in the base plate at those positions occupied by marker pins in the underlying datum plate. Both datum plate, and base plate with dowel pins inserted are then removed from the carrier plate, and the base plate flipped over and remounted to the opposite side of the carrier plate upright, over a dental impression containing freshly poured liquid die stone, and pushed downwards so that the base plate contacts the impression. The dowel pins thus protrude into the liquid die stone, and are thereby secured in the cast dental arch when the die stone hardens.
In a second version disclosed in Kiefer, a transparent base plate having pre-formed blind holes on one side, and depressions on the opposite side of the plate aligned with the holes, for receiving colored marking ink, is placed over a dental impression. Those depressions are locations where dowel pins are desired are then marked with ink, and dowel pins inserted in the corresponding blind holes.
All of the prior art methods for locating dowel pins in a cast dental arch, including those disclosed in Zeiser and Kiefer, require the determination of dowel pin locations to be made with impressions fixed and stabilized in position relative to reference points on a specially designed impression mounting or carrier plate. Prior art systems also require that a fixed relationship exist between the base plate and mounting plate. This requirement demands elaborate procedures for stabilizing, "boxing," i.e., providing an enclosure for holding liquid die stone and supporting a base plate in an established relationship to the impression. Also, all undercut areas of the negative dental impression, which has a irregular shape that varies in size from one patient to another, must be "blocked out" or trimmed away from the impression. This is to prevent liquid die stone from being trapped in irregular regions of the impression, which would hinder removal of a cast dental arch from the impression.
The present invention was conceived of to provide an improved method and apparatus for producing dental models that overcome certain limitations of prior art systems.